For efficient production of livestock feed, it is preferable to be able to supply different feed components in unprocessed form to hoppers of a mill, using metering means or premeasuring the quantities to effect the desired mixture, rather than having to mill the feed components separately and then have a separate mixing step.
Ordinarily, it is desirable that nothing be done to the grain in a mill beyond cracking it. In feeding cows, for example, it is undesirable to have the grain finely ground, as a coarse grain mix stimulates rumination or cud chewing and results in the desired high butterfat levels in the cows' milk.
Accordingly, there is a problem in producing a mill which will mill different feed components in optimum fashion. For example, it is frequently desirable to produce a mixture of corn and dry oats or barley. Not only does corn require a larger spacing between rollers than oats or barley, but furthermore the grooves in the rollers should preferably be wider and deeper than for oats or barley in order to achieve the perfect crack.
The main way of dealing with this problem in the past, apart from milling the different components at different times and mixing them subsequently, has been to provide completely separate mills fed by separate hoppers, with the output from the separate mills being mixed by funnelling them into a common output. These separate mills may have differently grooved rollers.
Another way developed to deal with the problem is that described in Canadian Pat. No. 1,075,658 (Keil). Keil describes a "split roll" roller mill, in which there are two identical opposing rollers, each roller having one section of one diameter and another section of a slightly smaller diameter, the smaller diameter section also having wider and deeper grooves for grinding. Thus two different roller grinding surfaces and spacings are provided for any given mill setting. Although certainly a useful improvement, Keil's mill obviously does not permit independent adjustment of the two different roller spacings.